1. Field of the Invention
This invention relates to a lignin-based melamine formaldehyde resin suitable for use as an adhesive binder in composition board applications.
2. Description of the Prior Art
Lignosulfonates have adhesive characteristics to a limited extent and numerous publications teach using lignosulfonate as an adhesive or the addition of lignosulfonates to adhesives such as phenolic adhesives, as an extender or a replacement of a resin constituent. In U.S. Pat. No. 2,849,314 lignosulfonate is treated with ammonia and then reacted with an aldehyde, an amine, or a hydroxy aromatic compound to obtain a thermosetting resin. U.S. Pat. Nos. 2,786,820 and 3,296,159 likewise disclose a reaction of lignin with aldehyde in preparation of water soluble thermosetting adducts which may be used as adhesives or added to phenol-formaldehyde adhesives. Using lignosulfonate to replace a portion of the phenol in preparation of phenolic adhesives is taught in U.S. Pat. Nos. 2,395,375; 3,658,638; 3,597,375; 3,677,884; 3,886,101; 3,940,352; 3,956,207; and 3,931,072. Different molecular weight lignosulfonate were also used as extenders in phenolic adhesives as reported by Julius Benko in Volume 44, No. 12, pages 849-854 of the TAPPI (December, 1961) published by the Technical Association of the Paper and Pulp Industry. Further, water insoluble lignin is commonly added as fillers in phenol-formaldehyde plywood adhesives.
The lignosulfonate containing binder compositions of the prior art suffer from a number of serious drawbacks. First of all, the polymerized lignin materials contain fewer active sites on the ring structure to form linkages. A lignin resin will, therefore, be weaker than a fully reacted typical resin of the known type. In addition, the sulfonic acid groups present in the lignosulfonates result in inferior water resistance of the cured resin. While some of the lignosulfonate containing resins based on phenolics have somewhat better water resistance, these products require higher press temperatures and longer press cycles--both conditions undesirable and sometimes even intolerable in composition board plants. Many composition board processes utilizing lignin based adhesives also require additional board treatments such as post-curing at elevated temperatures. In general, all prior art lignin containing binders possess the major drawbacks of being slow curing and requiring high press temperatures.